arXiv subjects

Christoph Heil

Publications and source records attributed to Christoph Heil.

42 records · Page 3Linked to original sources

arXiv2015

Influence of bonding on superconductivity in high-pressure hydrides

The recent reports on high-temperature superconductivity above 190 K in hydrogen sulfide at 200 GPa pressure, exceeding all previously discovered superconductors, has greatly invigorated the interest in dense hydrogen-rich solids. In this paper, we investigate a possible way to optimize the critical temperature in these compounds using first-principles linear-response calculations. We construct hypothetical alchemical atoms to smoothly interpolate between elements of the chalcogen group and study their bonding and superconducting properties. Our results show that the already remarkable critical temperatures of H$_3$S could be improved even further by increasing the ionic character of the relevant bonds, i.e. replacing sulfur partially with more electronegative elements.

cond-mat.supr-con↗

arXiv2014

Effect of the iron valence in the two types of layers in LiFeO$_2$Fe$_2$Se$_2$

We perform electronic structure calculations for the recently synthesized iron-based superconductor LiFeO$_2$Fe$_2$Se$_2$. In contrast to other iron-based superconductors, this material comprises two different iron atoms in 3$d^5$ and 3$d^6$ configurations. In band theory, both contribute to the low-energy electronic structure. Spin-polarized density functional theory calculations predict an antiferromagnetic metallic ground state with different moments on the two Fe sites. However, several other almost degenerate magnetic configurations exist. Due to their different valences, the two iron atoms behave very differently when local quantum correlations are included through the dynamical mean-field theory. The contributions from the half-filled 3$d^5$ atoms in the LiFeO$_2$ layer are suppressed and the 3$d^6$ states from the FeSe layer restore the standard iron-based superconductor fermiology.

cond-mat.supr-con↗

arXiv2014

Accurate bare susceptibilities from full-potential $\textit{ab initio}$ calculations

Electronic susceptibilities are a very popular tool to study electronic and magnetic properties of materials, both in experiment and theory. Unfortunately, the numerical evaluation of even the bare susceptibility, which depends on the computation of matrix elements and sums over energy bands, is very work-intensive and therefore various approximations have been introduced to speed up the calculations. We present a reliable and efficient implementation to compute static as well as dynamic bare susceptibilities based on full-potential density functional theory (DFT) calculations. Based on the exact results we will assess the accuracy of replacing the matrix elements with a constant and the impact of truncating the sum over the energy bands. Results will be given for representative and topical materials, such as Cr, a classical transition metal, as well as for FeSe and LaFeAsO, examples of iron-based superconductors.

cond-mat.supr-con↗

arXiv2012

Steady-state spectra, current and stability diagram of a quantum dot: a non-equilibrium Variational Cluster Approach

We calculate steady-state properties of a strongly correlated quantum dot under voltage bias by means of non-equilibrium Cluster Perturbation Theory and the non-equilibrium Variational Cluster Approach, respectively. Results for the steady-state current are benchmarked against data from accurate Matrix Product State based time evolution. We show that for low to medium interaction strength, non-equilibrium Cluster Perturbation Theory already yields good results, while for higher interaction strength the self-consistent feedback of the non-equilibrium Variational Cluster Approach significantly enhances the accuracy. We report the current-voltage characteristics for different interaction strengths. Furthermore we investigate the non-equilibrium local density of states of the quantum dot and illustrate that within the variational approach a linear splitting and broadening of the Kondo resonance is predicted which depends on interaction strength. Calculations with applied gate voltage, away from particle hole symmetry, reveal that the maximum current is reached at the crossover from the Kondo regime to the doubly-occupied or empty quantum dot. Obtained stability diagrams compare very well to recent experimental data [Phys. Rev. B, 84, 245316 (2011)].

cond-mat.str-el↗

arXiv2012

Indirect evidence for strong coupling superconductivity in FeSe under pressure from first-principle calculations

Whether the pairing mechanism for superconductivity in iron-based superconductors is of itinerant or local character is a question still debated on. In order to investigate the influence of Fermi surface nesting, we calculate from first-principles the static susceptibility of FeSe under applied pressures, both hydrostatic and non-hydrostatic. We show that the electronic band structures are highly sensitive to the way pressure is applied and confront our theoretical results with conclusions drawn from experiments. Given that the critical temperature of FeSe is quite universal as function of pressure, this is clear evidence that in FeSe the evolution of Fermi surface nesting cannot account for the evolution of the critical temperature with pressure. Hence we argue that the pairing in iron-chalcogenide compounds should be of strong-coupling spin fluctuation origin.

cond-mat.supr-con↗

arXiv2012

Strong coupling expansion for the Bose-Hubbard and the Jaynes-Cummings lattice model

A strong coupling expansion, based on the Kato-Bloch perturbation theory, which has recently been proposed by Eckardt et al. [Phys. Rev. B 79, 195131] and Teichmann et al. [Phys. Rev. B 79, 224515] is implemented in order to study various aspects of the Bose-Hubbard and the Jaynes-Cummings lattice model. The approach, which allows to generate numerically all diagrams up to a desired order in the interaction strength is generalized for disordered systems and for the Jaynes-Cummings lattice model. Results for the Bose-Hubbard and the Jaynes-Cummings lattice model will be presented and compared with results from VCA and DMRG. Our focus will be on the Mott insulator to superfluid transition.

cond-mat.mes-hall↗